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pythonregexlookbehindnegative-lookbehind

Python regex for parsing version strings and look behind fixed width

I'm trying to write a Python library to parse our version format strings. The (simplified) version string format is as follows:

<product>-<x>.<y>.<z>[-alpha|beta|rc[.<n>]][.<extra>]][.centos|redhat|win][.snb|ivb]

This is:

  • product, ie foo
  • numeric version, ie: 0.1.0
  • [optional] pre-release info, ie: beta, rc.1, alpha.extrainfo
  • [optional] operating system, ie: centos
  • [optional] platform, ie: snb, ivb

So the following are valid version strings:

1) foo-1.2.3
2) foo-2.3.4-alpha
3) foo-3.4.5-rc.2
4) foo-4.5.6-rc.2.extra
5) withos-5.6.7.centos
6) osandextra-7.8.9-rc.extra.redhat
7) all-4.4.4-rc.1.extra.centos.ivb

For all of those examples, the following regex works fine:

^(?P<prod>\w+)-(?P<maj>\d).(?P<min>\d).(?P<bug>\d)(?:-(?P<pre>alpha|beta|rc)(?:\.(?P<pre_n>\d))?(?:\.(?P<pre_x>\w+))?)?(?:\.(?P<os>centos|redhat|win))?(?:\.(?P<plat>snb|ivb))?$

But the problem comes in versions of this type (no 'extra' pre-release information, but with os and/or platform):

8) issue-0.1.0-beta.redhat.snb

With the above regex, for string #8, redhat is being picked up in the pre-release extra info pre_x, instead of the os group.

I tried using look-behind to avoid picking the os or platform strings in pre_x:

...(?:\.(?P<pre_x>\w+))?(?<!centos|redhat|win|ivb|snb))...

That is: 

^(?P<prod>\w+)-(?P<maj>\d).(?P<min>\d).(?P<bug>\d)(?:-(?P<pre>alpha|beta|rc)(?:\.(?P<pre_n>\d))?(?:\.(?P<pre_x>\w+))?(?<!centos|redhat|win|ivb|snb))?(?:\.(?P<os>centos|redhat|win))?(?:\.(?P<plat>snb|ivb))?$

This would work fine if Python's standard module re could accept variable width look behind. I would rather try to stick to the standard module, rather than using regex as my library is quite likely to be distributed to a large number machines, where I want to limit dependencies.

I've also had a look at similar questions: this, this and this are not aplicable.

Any ideas on how to achieve this?

My regex101 link: https://regex101.com/r/bH0qI7/3

[For those interested, this is the full regex I'm actually using: https://regex101.com/r/lX7nI6/2]

Solution

  • You need to use negative lookahead assertion to make (?P<pre_x>\w+) to match any except for centos or redhat.

    ^(?P<prod>\w+)-(?P<maj>\d)\.(?P<min>\d)\.(?P<bug>\d)(?:-(?P<pre>alpha|beta|rc)(?:\.(?P<pre_n>\d))?(?:\.(?:(?!centos|redhat)\w)+)?)?(?:\.(?P<os>centos|redhat))?(?:\.(?P<plat>snb|ivb))?$

    DEMO